Ultrasound Evaluation and Guided Injection of the Subscapularis and Serratus Anterior Muscles Between the Scapula and the Thoracic Cage

Most subscapularis and serratus anterior muscles lie between the scapula and the thoracic cage. Evaluation of this area in patients with scapulothoracic dyskinesis, snapping scapular syndrome, or interscapular pain can provide valuable information to clinicians. However, ultrasound scanning of pathologies in this area is hindered by anatomical limitations. In this study, we described a simple patient setup position and scanning method for ultrasound evaluation and guided intervention of the subscapularis and serratus anterior muscles between the scapula and thoracic cage.

U ltrasound is widely used to evaluate the shoulder and periscapular structures.Evaluating and treating the periscapular muscles and fascia is important because these structures help stabilize the scapula and position the glenohumeral fossa for overhead activities. 1,2However, when examining periscapular structures, there is a blind spot for ultrasound: the ventral surface of the scapula, which is blocked anteriorly by the thoracic cage and posteriorly by the scapula.The acoustic shadow cast by the scapula significantly limits the sonographic view of the musculature deep to the scapula and traditionally requiring magnetic resonance imaging or computed tomography for assessment. 3lthough ultrasound-guided injections offer safety and accuracy, interventions in the ventral surface of the scapula primarily depend on surface anatomy guidance and sustain certain risks due to a lack of adequate evaluation and proximity to the lung. 4Ultrasound evaluation and treatment of the axilla (lateral border of the scapula) can only access the mid-to-distal portions of the subscapularis and serratus anterior muscles and are constrained by the patient's ability to perform adequate shoulder forward flexion. 5n this article, we describe a new scanning method for evaluating structures on the ventral side of the scapula with the relevant sonoanatomy and the subsequent ultrasound-guided intervention of the ventral scapular subscapularis and the serratus anterior muscles.

Technique
To scan the ventral side of the scapula, the patient was instructed to lie in a side-lying position with the scapula examined against the bed and the elbow kept at the side of the body or slightly anterior to it.The examiner could adjust the trunk position to make the medial border of the scapula protrude from the thoracic cage, creating a space for ultrasound examination.
The examiner placed the curved transducer across the space between the medial scapular border and posterior wall of the thorax, and ensuring the direction of the probe was perpendicular to the medial scapular edge (Figure 1).For optimal visualization, the depth of focus should be set at 6 cm.The transducer scanned from the superomedial angle of the scapula to the inferomedial angle.The sono-anatomy differs in these two angles, and you can use the spine of the scapulae as an anatomic reference during scanning.
Hyperechoic fascia lines are visible between the muscles.The most superficial layer comprises the trapezius muscle and its fascia, followed by the levator scapulae and rhomboids (from superior to inferior), serratus anterior, subscapularis muscle, and the anterior surface of the scapula.On the opposite side of the scapula, ribs and lung pleura become visible (Figure 2 and Figure 3).The examiner can move the transducer from the superior-medial border to the inferior-medial border of the scapula (Supplemental Video S1).For dynamic scanning of the subscapularis muscle, the patient can perform humeral internal rotation simultaneously.
If an ultrasound-guided injection is necessary, an out-of-plane injection can be administered in the same side-lying position.This should be done after achieving a clear visualization of the border of the ribs, pleura, the spinal accessory nerve, and the dorsal scapular neurovascular bundle between the rhomboids and serratus anterior (Supplemental Video S2).

Discussion
The scapulothoracic joint encompasses the muscles and fascia between the anterior surface of the scapula and posterior surface of the thoracic wall.Williams et al proposed a three-layer model (superficial, intermediate, and deep) for the scapulothoracic joint. 6In this three-layer model, the deep layer comprises the serratus anterior and subscapularis, forming a cushion between the scapula and thoracic wall that glides on the serratti fascia.In patients with scapulothoracic dyskinesis, tension around the scapula is disrupted, leading to increased friction in the scapulothoracic joint.Consequently, the cushion layer becomes compromised.
Historically, clinicians have relied on magnetic resonance imaging and computed tomography to examine the deep layers of the scapulothoracic joint.Due to the bony anatomy in this region, even if the patient can perform full internal rotation in the Crass position, ultrasound still offers a limited view, potentially leading to overlooked pathologies, such as muscle tears or space-occupying lesions, like elastofibroma dorsi or osteochondromas. 7Trigger point injection of the subscapularis and serratus anterior muscles at the ventral side of the scapula, Figure 2. A and B, Sonoanatomy of the ventral surface of the scapula cephalic to the spine of the scapula (Probe position: Blue square in Figure 1).Yellow lines denote the borders of muscles and bone.The green circle is the spinal accessory nerve.The red circle is the dorsal scapular neurovascular bundles.SA, serratus anterior.Figure 3.A and B, Sonoanatomy of the ventral surface of the scapula caudal to the spine of the scapula (Probe position: Orange square in Figure 1).Yellow lines denote the borders of muscles and bone.The red circle is the dorsal scapular neurovascular bundles.SA, serratus anterior.
guided by surface anatomy, has been introduced by Cassius et al.However, due to the close proximity to the lung and the variation of thoracic shape among individuals, intervention in this area without guidance could lead to serious complications. 4 Our proposed approach allows for the assessment of most of the middle and inferior bundles of the serratus anterior and subscapularis muscles.Both of the serratus anterior and subscapularis are multipennate muscles, necessitating a thorough examination through scanning is necessary, as injuries may only occur in isolated parts of the pennate structure.Hyperechoic changes with disruption of the fascia line between the serratus anterior and subscapularis are common in patients with symptomatic scapulothoracic dyskinesis.Comparison with the opposite, less pathological scapulothoracic joint may offer additional information to facilitate making the diagnosis (Figure 4).
In addition, shoulder adduction and internal rotation are commonly observed in patients with spasticity following cerebrovascular events.The proposed method for ultrasound-guided botulinum toxin injections into the subscapularis muscle can be performed safely and efficiently using this proposed method comparing to the lateral approach. 5This method proves especially valuable when patients struggle to achieve sufficient anterior shoulder flexion, a common challenge among stroke patients.

Conclusion
Ultrasound scanning from the medial scapula with the patient in a side-lying position offers an additional approach for both evaluating and treating scapulothoracic joints.This method can be used in patients experiencing with scapulothoracic dyskinesis, interscapular pain, or spasticity following a stroke.

Figure 1 .
Figure 1.A, The patient is in a side-lying position with the examined scapula against the bed.B, Depicted anatomy of the ventral surface of the scapula (the rhomboids have been removed).